The attacked player will receive the 3 points of damage in the form of infect which makes damage a player takes into poison counters, so if you attack for 3 unblocked infect damage the opponent will take 3 poison counters. and once they have ten counters they lose the game.
3 poison counters...
That would be if it's your creature attacking and that there is no creature blocking your creature.
actually that really depends. If the damaging creature has infect then the above are correct, and the defending player gets 3 poison counters if the creature's power is 3. If the creature has poisonous X, then they get X poison counters. If the creature has neither of these abilities and hands out poison counters then the card will specify how many counters they receive.
I was going to say something after I saw the first responses, but then I saw squire1 say it. Poison is not just from Infect. When you talk about Poison Counters you need to specify what they are coming from, whether they are from a creature with Infect or Poison X.